1. Technical Field
The present invention relates to wireless communications and, more particularly, to mobility of wireless packet-data-communication devices.
2. Description of Related Art
More people than ever are using mobile nodes, such as cellular telephones and personal digital assistants, to engage in wireless communications. In addition to using these mobile nodes for voice communication, people are using them to engage in packet-data communication via wireless networks, such as wireless wide area networks (WWANs) and wireless local area networks (WLANs). As examples, this packet-data communication may involve e-mail, web browsing, receipt of streaming media, voice-over-Internet Protocol (VoIP) sessions, video sessions, instant messaging sessions, and other data services via these wireless networks.
Some of these data sessions, such as VoIP sessions for example, may be set up using the Session Initiation Protocol (SIP) and conducted using the real-time transport protocol (RTP), relevant aspects of which are described in “SIP: Session Initiation Protocol,” RFC 3261 (June 2002) and “RTP: A Transport Protocol for Real-Time Applications,” RFC 1889 (January 1996), respectively, which are incorporated herein by reference. In addition to these protocols, mobile nodes may use other signaling and bearer protocols, and other communication protocols.
As indicated by the term mobile node, these devices are mobile, typically designed to maintain or frequently re-establish packet-data connectivity with a network device via a nearby radio transceiver, such as may be found in a WWAN base station or a WLAN access point. Such a device may provide an interface for the mobile node between the transceiver and a packet-data network, and may be referred to as a mobility agent, though other names may be used as well, such as gateway, packet data serving node (PDSN), or network access server (NAS).
Furthermore, it is often the case that different radio transceivers are associated with different mobility agents, though each mobility agent may provide connectivity with the same packet-data network (perhaps via one or more additional networks). As a result, when a mobile node moves from communicating with a first radio transceiver to communicating with a second radio transceiver, the mobile node may also be required to switch from accessing the packet-data network via a first mobility agent associated with the first radio transceiver to accessing the packet-data network via a second mobility agent associated with the second radio transceiver.
This move by the mobile node from communicating with the first radio transceiver to communicating with the second radio transceiver, and thus from communicating with the first mobility agent to communicating with the second mobility agent, typically involves the mobile node switching from using a first network address, such as a first Internet Protocol (IP) address, assigned to the mobile node by or via the first mobility agent, to using a second network address, assigned to the mobile node by or via the second mobility agent.
This transition may be handled in several different ways. One way is by using IP, which may also be known as Simple IP, relevant aspects of which are described in “Internet Protocol,” RFC 791 (September 1981), which is incorporated herein by reference. In this case, the mobile node, upon transitioning to communication with the second mobility agent, would request an IP address from or via that mobility agent, perhaps using the Dynamic Host Configuration Protocol (DHCP), relevant aspects of which are described in “Dynamic Host Configuration Protocol,” RFC 2131 (March 1997) and “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” RFC 3315 (July 2003), which are incorporated herein by reference. The mobile node may then use that address for packet-data communication.
In this scenario, the mobile node would likely not be able to keep a communication session active when switching to the second mobility agent, since other nodes on the packet-data network would not yet know the mobile node's second IP address. Along those lines, the mobile node would need to re-register the new address with any entity maintaining contact information for the mobile node, such as a SIP proxy. In the context of a wireless network implementing simple IP, handoffs from one mobility agent to another can thus be costly in terms of communication disruption, not to mention battery power needed to effect the transition.
In order to address certain limitations of Simple IP, including the communication disruption described above, and to handle the mobility of packet-data-communication devices generally, a protocol known as Mobile IP has been and is still being developed, relevant aspects of which are described in “IP Mobility Support for IPv4,” RFC 3344 (August 2002) and “Mobility Support in IPv6,” RFC 3775 (June 2004), which are incorporated herein by reference.
In Mobile IP, mobile nodes are able to maintain a semi-permanent IP address, called a home address, to which other nodes on the Internet may transmit packets regardless of the mobile node's current point of attachment to the Internet. Furthermore, mobile nodes have a home network, on which resides a mobility agent known—with respect to the mobile node—as a home agent. When packets are transmitted to a mobile node's home address, those packets will be routed by normal IP routing mechanisms to the mobile node's home network.
When the mobile node is attached to its home network, it will receive these packets as any IP node would. When the home agent is attached to another network, known—again with respect to the mobile node—as a foreign network, the home agent will intercept these packets on the home network and forward them to an address which the mobile node has registered with the home agent. This address is known as a care-of address, and may be an IP address assigned to the mobile node on the foreign network, or it may be an IP address of a mobility agent on that foreign network, known—with respect to the mobile node—as a foreign agent, which would then forward the packets to the mobile node.
In a Mobile IP scenario, then, a mobile node may maintain connectivity using its home address, whether involved at the time in a communication session or not, either by connecting to its home network, or by conducting Mobile IP registrations and re-registrations, updating its home agent with the IP address of the particular foreign agent with which the mobile node is connected at that time. Whether a particular wireless network implements Simple IP, Mobile IP, or some combination thereof, and whether that network implements IPv4, IPv6, or some combination thereof, it may be the case that mobile nodes are programmed to conduct registrations or re-registrations with whichever mobility agent is associated with the radio transceiver from which the mobile node is currently receiving the strongest radio signal.
In Simple IP, or in Mobile IP when the mobile node is connected to its home network, these registrations may simply take the form of the mobile node establishing a link-layer connection with a mobility agent and then requesting an IP address using a protocol such as DHCP. In Mobile IP, when the mobile node is connected to a foreign network, these registrations may take the form of the mobile node establishing a link-layer connection with the mobility agent (i.e., the foreign agent) and then exchanging registration request and reply messages via that mobility agent with the mobile node's home agent, to register the mobility agent's IP address as the care-of address for the mobile node.
Whether Simple IP, Mobile IP, or another protocol is implemented in a wireless network, there are undesirable consequences when a mobile node conducts frequent registrations and re-registrations. Some of these consequences include draining the mobile node's supply of battery power, lowering the quality of service that a user of the mobile node experiences, consuming network resources, and potentially blocking circuit-switched voice calls while the mobile node repeatedly makes “data calls” to conduct the re-registrations. And there may be other undesirable consequences as well.
Frequent registrations and re-registrations may occur when a mobile node is located on or near a boundary between the coverage areas of two radio transceivers, where each radio transceiver is associated with a different mobility agent, or in an area of overlap between two such coverage areas; in that situation, the respective radio signals that the mobile node may be receiving from the radio transceivers may each be sufficiently strong to support packet-data communication, though one radio signal may be slightly stronger than the other.
Furthermore, even while relatively stationary, the mobile node may observe frequently-changing strength of signals from the two radio transceivers due to atmospheric conditions, physical obstructions, terrain, or other reasons. Thus, even though the signals may each be strong enough to support packet-data communication, the particular signal that is the strongest at any given moment may change repeatedly during a particular time period.
In these situations, a mobile node programmed to conduct a registration or re-registration with the mobility agent associated with the radio transceiver from which the mobile node is currently receiving the strongest signal may repeatedly conduct registrations and re-registrations back and forth between the mobility agents respectively associated with these radio transceivers, in order to maintain VoIP and other packet-data services. Thus, the undesirable consequences described above, and perhaps other undesirable consequences, may occur unnecessarily, as the signals from both transceivers are each sufficient to support packet-data communication.